Evolved universal terrestrial radio access (E-UTRA) is expected to use a multiple-access scheme known as single-carrier (SC) frequency division multiple access (SC-FDMA) on the uplink. With such a scheme, users connected to the same cell can completely avoid mutual interference by transmitting on different subcarriers and/or different timeslots, (or transmission timing intervals (TTIs)). In one typical way of operating the system, a user could be assigned a set of subcarriers on a long term basis, and be scheduled through fast signaling at different TTIs. A base station controls the allocation of resources for a cell or a set of cells. The base station can easily coordinate the transmissions of users connected to these cells to avoid mutual interference. However, a base station does not directly control the transmissions from users connected to other cells. If transmissions from these other-cell users are received at a significantly high level at the base station antennas, they create interference. Thus, performance in the cell suffers.
The inter-cell interference problem is particularly significant when a user is situated at a location such that its path losses to two (or more) cells controlled by different base stations are approximately the same. Such users are often referred to as “boundary users.”
Boundary users create a more acute inter-cell interference problem because the received signal levels are approximately the same between the cell they are connected to (serving cell) and the cell(s) they are not connected to (i.e., non-serving cell(s)). Therefore, at the non-serving cell, the signal from a boundary user is likely to be relatively strong. Since the non-serving cell is not controlling the subcarriers and time of transmission from the boundary user, the likelihood of collision with other users is high. Thus, for these users, either more retransmissions are needed or the modulation/coding scheme must be more conservative, resulting in decreased throughput.
FIG. 1 shows one conventional approach that is being used to segregate the set of resources that can be used in adjacent cells controlled by different Node-Bs NB1, NB2 and NB3 in a multi-cell wireless communication system 100. The Node-Bs NB1, NB2 and NB3 may be evolved Node-Bs (eNodeBs). The letters A, B, C and D shown in FIG. 1 represent areas of sectors 105, 110 and 115 of different cells, where blocks of resources, (e.g., subcarriers, timeslots, orthogonal codes, etc.), are available for users in the respective cell sectors. There are typically three sectors in a cell, only one of which is shown in FIG. 1 for each Node-B NB1, NB2 and NB3.
As shown in FIG. 1, resource block B can only be used in the cell sector 110 from NB2, resource block C can only be used in the cell sector 105 from NB1, and resource block D can only be used in the cell sector 115 from NB3. The arrows shown in FIG. 1 point in the direction of the main lobe of the antennas of the respective Node-Bs NB1, NB2 and NB3. Users that transmit on different resource blocks do not interfere with each other (“orthogonal” transmissions). For example, if the resource blocks consist of different sets of frequencies, the users transmitting on these different sets of frequencies do not interfere with each other.
Typically, a specific cell is first assigned to a user based on path loss considerations. A block of resources available in a particular sector of the specific cell is then assigned to the user based on path loss considerations.
This prior art approach prevents interference from boundary users because two users that are located within the inter-Node-B border area, but connected to different Node-Bs, are using different resource blocks, and hence do not interfere with each other. However, this approach incurs a severe penalty in terms of overall spectrum efficiency, since certain blocks of resources cannot be reused in every cell. Furthermore, this approach fails to exploit the intra-cell user orthogonality that is present with orthogonal FDMA (OFDMA) and SC-FDMA systems in particular, and does not result in the highest possible capacity. Because, as described above, certain resource blocks can only be used in specific cell sectors, the total number of users that can be served in the whole system for a given amount of resource blocks, (i.e., a given amount of spectrum), is less than what it could be if the resource blocks could be used in all sectors (or more sectors). Thus, the capacity is not as high as it could be.
It would therefore be beneficial if a resource and cell assignment method and apparatus existed that was not subject to the limitations of the existing prior art.